Electric power steering control system

ABSTRACT

Conventionally, there was a problem that in cases when it is difficult to place a temperature sensor in the vicinity of a heat-generating portion, overheating could not be prevented. Disclosed is an electric power steering control system including a torque sensor for detecting steering force of a steering system; a motor current determination unit for determining the motor current based on the steering force detected by the torque sensor; a temperature sensor for detecting an ambient temperature; a coefficient setting unit for setting a coefficient in accordance with the detected temperature obtained by means of the temperature sensor; a motor current detection unit for detecting a motor current being passed to a motor; a maximum current limit value calculation unit for calculating a maximum current limit value based on the detected current and the coefficient; a current limiting unit for selecting the smaller value between the motor current determined by the motor current determination unit and the maximum current limit value calculated and outputting this as a target current; and a motor current control section for passing the target current to the motor in such a way that the motor current is equal to the detected current, whereby a motor current limit being set according to temperature is applied.

[0001] This application is based on Application No. 2001-169669, filedin Japan on Jun. 5, 2001, the contents of which are hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to electric power steering forreducing a required steering force in a steering system by means ofmechanical power of a motor. More particularly, the present inventionrelates to an electric power steering control system for preventingoverheating of the motor and a motor drive circuit.

[0004] 2. Description of the Related Art

[0005] As an example of an art having an objective of preventingoverheating of a motor and a motor drive circuit, there exists anelectric power steering device described in Japanese Patent ApplicationLaid-open No. 11-59444.

[0006] According to the above-mentioned conventional art, a heat sensoris provided on a periphery of the motor or the motor drive circuitportion, a motor current limit value is calculated based on a heatingvalue estimated from a temperature detected by the temperature sensorand a current of the motor, and the motor current is limited by means ofthis calculated motor current limit value. Accordingly, the overheatingof the motor is prevented.

[0007] According to the above-mentioned conventional art, the heatsensor is provided to a vicinity of a location generating heat, andcontrol for preventing overheating is realized by means of directlydetecting an ambient temperature of the location generating heat.However, in an actual system there are cases when it is difficult, forreasons of construction and cost, to place the temperature sensor in thevicinity of the heat-generating part. In such cases, prevention ofoverheating becomes problematic in the conventional art. Furthermore, ina case when there exists a plurality of heat-generating parts (or partsat which it is desirable to estimate the temperature thereof), aplurality of temperature sensors and I/F circuits need to be installed.Therefore, there was a problem that it was disadvantageous in terms ofcost and miniaturization.

SUMMARY OF THE INVENTION

[0008] The present invention has been made to solve the problemsdescribed above. Therefore, an object of the present invention is toobtain an electric power steering control system capable of achievingoverheating prevention even without a temperature sensor being providedto a vicinity of a heat-generating location.

[0009] An electric power steering control system according to thepresent invention comprises a motor for adding a steering assistingforce to a steering system; a steering force detection section fordetecting steering force in the steering system; a motor currentdetermination section for determining a motor current based on at leastthe steering force detected by means of the steering force detectionsection; a temperature detection section for detecting an ambienttemperature; a coefficient setting section for setting a coefficient inaccordance with a detected temperature obtained by means of thetemperature detection section; a motor current determination section fordetecting a current being passed to the motor; a maximum current limitvalue calculation section for calculating a maximum current limit valuebased on the detected current detected by means of the motor currentdetection section and the coefficient set by the coefficient settingsection; a current limiting section for selecting and outputting as atarget current the smaller of either the motor current determined bymeans of the motor current determination section or the maximum currentvalue calculated by means of the maximum current limit value calculationsection; a motor current control section for passing the target currentto the motor such that the target current becomes equal to the detectedcurrent being detected by the motor current detection section.

[0010] An electric power steering control system according to thepresent invention comprises a motor for adding steering assistance forceto a steering system; a steering force detection section for detectingsteering force of the steering system; a motor current determinationsection for determining a motor current based on at least the steeringforce detected by means of the steering force detection section; atemperature detection section for detecting an ambient temperature; atimer for measuring time from when predetermined conditions areestablished; a control temperature calculation section for calculating acontrol temperature based on the temperature detected by the temperaturedetection section and the time measured by the timer; a coefficientsettling section for setting a coefficient based on the controltemperature calculated by the control temperature calculation section; amotor current detection section for detecting a current being passed tothe motor; a maximum current limit value calculation section forcalculating a maximum current limit value based on a current detected bythe motor current detection section and the coefficient set by thecoefficient setting section; a current limiting section for selectingthe smaller value between the motor current determined by the motorcurrent determination section and the maximum current limit valuecalculated by the maximum current limit value calculation section, andoutputting this as a target current; and a motor current control sectionfor passing the target current to the motor in such a way that the motorcurrent is equal to the current detected by the motor current detectionsection.

[0011] An electric power steering control system according to thepresent invention comprises a motor for adding steering assistance forceto a steering system; a steering force detection section for detectingsteering force of the steering system; a motor current determinationsection for determining a motor current based on at least the steeringforce detected by means of the steering force detection section; atemperature detection section for detecting an ambient temperature; atimer for measuring time from when predetermined conditions areestablished; a control temperature calculation section for calculating acontrol temperature based on the temperature detected by the temperaturedetection section and the time measured by the timer; a coefficientsetting section for setting a coefficient based on the controltemperature calculated by the control temperature calculation sectionand the temperature detected by the temperature detection section; amotor current detection section for detecting a current being passed tothe motor; a maximum current limit value calculation section forcalculating a maximum current limit value based on a current detected bythe motor current detection section and the coefficient set by thecoefficient setting section; a current limiting section for selectingthe smaller value between the motor current determined by the motorcurrent determination section and the maximum current limit valuecalculated by the maximum current limit value calculation section, andoutputting this as a target current; and a motor current control sectionfor passing the target current to the motor in such a way that the motorcurrent is equal to the current detected by the motor current detectionsection.

[0012] Further, in an electric power steering control system accordingto the present invention, the predetermined condition is that the keyswitch is on.

[0013] Further, an electric power steering control system according tothe present invention further comprises an engine rotation detectionsection for detecting the number of engine rotations, wherein thepredetermined condition is that the number of engine rotations detectedby the engine rotation detection section is greater than a predeterminedvalue.

[0014] Further, an electric power steering control system according tothe present invention further comprises a vehicle speed detectionsection for detecting a vehicle speed, wherein the predeterminedcondition is that the vehicle speed detected by the vehicle speeddetection section is above a predetermined value.

[0015] Further, in an electric power steering control system accordingto the present invention, the predetermined condition is that thesteering force detected by the steering force detection section isgreater than a predetermined value.

[0016] Further, in an electric power steering control system accordingto the present invention, the predetermined condition is that the motorcurrent is greater than a predetermined value.

[0017] Further, in an electric power steering control system accordingto the present invention, the coefficient setting unit sets thecoefficient in accordance with a detected temperature at the time ofactivation obtained by means of the temperature detection section.

[0018] Further, an electric power steering control system according tothe present invention further comprises a power supply holding sectionfor holding a power supply until the temperature detected by thetemperature detection section drops below a predetermined value afterthe key switch is turned off.

[0019] Further, an electric power steering control system according tothe present invention further comprises a power supply holding sectionfor holding a power supply until the temperature detected by thetemperature detection section drops below a predetermined value afterthe key switch is turned off, or until a duration of time having elapsedsince the key switch was turned off is measured and the elapsed durationof time becomes greater than a predetermined duration of time.

[0020] Additionally, in electric power steering control system accordingto the present invention, the control temperature calculation sectioncalculates the control temperature based on a temperature that is thetemperature detected by the temperature detection section and correctedby a correction amount set in accordance with characteristics ofself-generation of heat, and the duration of time measured by the timer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the accompanying drawings:

[0022]FIG. 1 is a diagram depicting a construction of an electric powersteering control system according to Embodiment 1 of the presentinvention;

[0023]FIG. 2 is a block diagram depicting a construction of the acontrol apparatus of the electric power steering control systemaccording to Embodiment 1 of the present invention;

[0024]FIG. 3 is a diagram depicting a control block of the controlapparatus of the electric power steering control system according to 1of the present invention;

[0025]FIG. 4 is a diagram depicting input and output characteristics ofa motor current determination unit of the control apparatus of theelectric power steering control system, according to Embodiment 1 of thepresent invention;

[0026]FIG. 5 is a diagram depicting a current limiting unit of thecontrol apparatus of the electric power steering control system,according to Embodiment 1 of the present invention;

[0027]FIG. 6 is a flow chart depicting processing of a calculation of amaximum current limit value for the control apparatus of the electricpower steering control system, according to Embodiment 1 of the presentinvention;

[0028]FIG. 7 is a diagram depicting a coefficient of a coefficientdetermining unit of the control apparatus in the electric power steeringcontrol system according to Embodiment 1 of the present invention;

[0029]FIG. 8 is a diagram depicting an example of results ofcalculations by a maximum current limit value calculation unit of thecontrol apparatus in the electric power steering control systemaccording to Embodiment 1 of the present invention;

[0030]FIG. 9 is a diagram depicting a control block of a controlapparatus in an electric power steering control system according toEmbodiment 2 of the present invention;

[0031]FIG. 10 is a flow chart depicting processing of a timer and acontrol temperature calculation unit of the control apparatus in theelectric power steering control system according to Embodiment 2 of thepresent invention;

[0032]FIG. 11 is a timing chart depicting operation of the timer and thecontrol temperature calculation unit of the control apparatus in theelectric power steering control system according to Embodiment 2 of thepresent invention;

[0033]FIG. 12 is a diagram depicting a block control of an electricpower steering control system according to Embodiment 3 of the presentinvention;

[0034]FIG. 13 is a flow chart depicting processing of a timer and acontrol temperature calculation unit of a control apparatus in theelectric power steering control system according to Embodiment 3 of thepresent invention;

[0035]FIG. 14 is a flow chart depicting processing of a coefficientsetting unit of the control apparatus in the electric power steeringcontrol system according to Embodiment 3 of the present invention;

[0036]FIG. 15 is a timing chart depicting operation of the timer, thecontrol temperature calculation unit and the coefficient setting unit ofthe control apparatus in the electric power steering control systemaccording to Embodiment 3 of the present invention;

[0037]FIG. 16 is a diagram depicting a coefficient of the coefficientsetting unit of the control apparatus in the electric power steeringcontrol system according to Embodiment 3 of the present invention;

[0038]FIG. 17 is a diagram depicting a control block of a controlapparatus of an electric power steering control system according toEmbodiment 4 of the present invention;

[0039]FIG. 18 is a flow chart depicting processing of a timer and acontrol temperature calculation unit of the control apparatus in theelectric power steering control system, according to Embodiment 4 of thepresent invention;

[0040]FIG. 19 is a timing chart of operations of the timer, the controltemperature calculation unit and a coefficient setting unit of thecontrol apparatus in the electric power steering control system,according to Embodiment 4 of the present invention;

[0041]FIG. 20 is a diagram depicting a control block of a controlapparatus of an electric power steering control system, according toEmbodiment 5 of the present invention;

[0042]FIG. 21 is a flow chart depicting processing of a timer and acontrol temperature calculation unit of the control apparatus in theelectric power steering control system, according to Embodiment 5 of thepresent invention;

[0043]FIG. 22 is a flow chart depicting processing of the timer and thecontrol temperature calculation unit of the control apparatus in theelectric power steering control system, according to Embodiment 5 of thepresent invention;

[0044]FIG. 24 is a diagram depicting a control block of a controlapparatus of an electric power steering control system according toEmbodiment 6 of the present invention;

[0045]FIG. 25 is a flow chart depicting processing of a timer and acontrol temperature calculation unit of the control apparatus in theelectric power steering control system, according to Embodiment 6 of thepresent invention;

[0046]FIG. 26 is a flow chart depicting processing of the timer and thecontrol temperature calculation unit of the control apparatus in theelectric power steering control system, according to Embodiment 6 of thepresent invention;

[0047]FIG. 27 is a timing chart depicting operations of the timer, thecontrol temperature calculation unit and the coefficient setting unit ofthe control apparatus in the electric power steering control system,according to Embodiment 6 of the present invention;

[0048]FIG. 28 is a flow chart depicting processing of a coefficientsetting unit of a control apparatus of an electric power steeringcontrol system according to Embodiment 7 of the present invention;

[0049]FIG. 29 is a diagram depicting a construction of a controlapparatus of an electric power steering control system according toEmbodiment 8 of the present invention;

[0050]FIG. 30 is a diagram depicting a construction of a power circuitof the control apparatus in the electric power steering control systemaccording to Embodiment 8 of the present invention;

[0051]FIG. 31 is a flow chart depicting processing of a micon in thecontrol apparatus in the electric power steering control systemaccording to Embodiment 8 of the present invention;

[0052]FIG. 32 is a flow chart depicting processing of a micon in acontrol apparatus in an electric power steering control system accordingto Embodiment 9 of the present invention;

[0053]FIG. 33 is a flow chart depicting processing of a timer and acontrol temperature calculation unit of a control apparatus in anelectric power steering control system according to Embodiment 10 of thepresent invention; and

[0054]FIG. 34 is a diagram depicting characteristics of a correctionamount of the control apparatus in the electric power steering controlsystem according to 10 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] Embodiment 1

[0056] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 1 of the present invention,making reference to the drawings. FIG. 1 is a diagram depicting aconstruction of the electric power steering control system according toEmbodiment 1 of the present invention. Note that the same referencenumerals in each of the drawings indicate the same or equivalent parts.

[0057] In FIG. 1, 1 is a handle; 2 is a torque sensor (i.e., steeringforce detection section) for detecting steering force of a steeringsystem; 3 is a vehicle speed sensor (i.e., vehicle speed detectionsection) for detecting a vehicle speed; 4 is an engine rotation signal[i.e., engine rotation sensor (i.e., engine rotation detection section)]for obtaining the number of engine rotations; 5 is a key switch; 6 is amotor for adding a steering assisting force to the steering system; 7 isa control apparatus which is installed in a deep area of a panel infront of a driver seat and a passenger seat of the car and whichcontrols the motor 6 based on information from the torque sensor 2, thevehicle speed sensor 3, the engine rotation signal 4, and the key switch5; 8 is a deceleration apparatus for transmitting output of the motor 6to the steering system and the like; 9 is a rack and pinion mechanismfor converting a rotational force into a horizontal force; 10 is a tierod for transmitting the horizontal force to a steering wheel to bedescribed hereinafter.

[0058]FIG. 2 is a diagram depicting a construction of a controlapparatus of the electric power steering control system according toEmbodiment 1 of the present invention; and 11 is the tie rod.

[0059] In FIG. 2, the torque sensors 2 through the control device 7 arethe same as those of FIG. 1. Reference numeral 12 is a battery forproviding electric power to the control device 7. Further, 71 is an I/Fcircuit for inputting a signal from the torque sensor 2; 72 is an I/Fcircuit for inputting a signal from the vehicle speed sensor 3; 73 is anI/F circuit for inputting the engine rotation signal 4; 74 is an I/Fcircuit for inputting a signal from the key switch 5; 75 is thetemperature sensor (the temperature detection section) for detecting thetemperature; 76 is an I/F circuit for inputting a signal from thetemperature sensor 75; 77 is a motor drive circuit for driving the motor6; 78 is a current detection circuit for detecting the current beingpassed to the motor 6; and 79 is a micon for performing control of theelectrical power steering.

[0060] Next, explanation will be made of an operation of the electricpower steering control system according to Embodiment 1 of the presentinvention, making reference to the drawings.

[0061]FIG. 3 is a diagram depicting a control block of processingcarried out by the micon inside the control apparatus of the electricpower steering control system according to Embodiment 1.

[0062] Explanation will now be made of the processing carried out by themicon 79. In FIG. 3, reference numeral 101 is a vehicle speed signaldetected by the vehicle speed sensor 3; 102 is a torque signal detectedby the torque sensor 2; and 103 is a temperature signal detected by thetemperature sensor 75.

[0063] Further, in FIG. 3, 104 is a motor current determination unit(i.e., the motor current determination section) for determining, basedon a vehicle speed signal 101 (VSP) and a torque signal 102 (TRQ), themotor current for assisting the steering force; 105 is a currentlimiting unit (i.e., the current limiting section) for applying a limitbeing the maximum current limit value (Limit), explained below, to themotor current (Im1) determined by the motor current determination unit104; 106 is a motor current control unit (i.e., the motor currentcontrol section) for passing the target current (Imt) indicated by thecurrent limiting unit 105 to the motor 6 in a controlled fashion suchthat the target current (Imt) is equivalent to the detected current(Imd) which is detected by an motor current detection unit explainedbelow; and 107 is the motor current detection unit (i.e., the motorcurrent detection section) for detecting the motor current andcorresponds to the current detection circuit 78 of FIG. 2.

[0064] Additionally, in FIG. 3, 108 is a coefficient setting unit (i.e.,the coefficient setting section) for setting a coefficient forcalculation of the maximum current limit value described below inaccordance with the temperature signal 103 (i.e., Temp); and 109 is amaximum current limit value calculation unit (i.e., the maximum currentlimit value calculation section) for calculating the maximum currentlimit value (i.e., Limit) based on the detected current (i.e., Imd)detected by means of the motor current detection unit 107 and thecoefficient which is determined by means of the coefficient setting unit108 (i.e., the maximum current limit value calculation section).

[0065]FIG. 4 is a diagram depicting input output characteristics of themotor current determination unit.

[0066] Here explanation will be made of the motor current determinationunit 104. The motor current determination unit 104 has the input andoutput characteristics shown in FIG. 4 and determines the motor current(Im1) in accordance with the torque (TRQ) and the vehicle speed (VSP).By having the characteristics shown in FIG. 4 a result is produced suchthat at a time of steering to the right the motor current is passed tothe right direction, so less steering force is required. Further, at atime of steering to the left, on the other hand, the motor current ispassed to the left direction, so less steering force is required.Additionally, altering the motor current in accordance with the vehiclespeed (VSP) produces a result that the steering assisting forceappropriate for each vehicle speed (ex, low vehicle speed through highvehicle speed) is generated.

[0067]FIG. 5 is a diagram depicting a construction of the currentlimiting unit.

[0068] Next, explanation will be made of the current limiting unit 105.The current limiting unit 105 has a construction as shown in FIG. 5, andselects and outputs as a target current (i.e., Imt) the smaller ofeither the motor current (i.e., Im1) determined by means of the motorcurrent determination unit 104 or the maximum current limit value (i.e.,Limit) calculated by means of the maximum current limit valuecalculation unit 109.

[0069]FIG. 6 is a flow chart depicting processing of the maximum currentlimit value calculation unit 109.

[0070] Next, explanation will be made of the maximum current limit valuecalculation unit 109. According to FIG. 6, at first when the controldevice 7 is activated at step 121, an initial value is set for themaximum current limit value (i.e., Limit). Next, at step 122 an thecurrent increase rate (kim) is calculated; at step 123 the currentincrease rate (kim) is added to the maximum current limit value (i.e.,Limit); and steps 122 to 123 are repeated thereafter.

[0071]FIG. 7 is a diagram depicting characteristics (i.e., coefficients)of the current increase rate of the maximum current limit valuecalculation unit.

[0072] At step 122 the characteristics of the current increase rate(kim) are as in FIG. 7 by the detected current of the motor (i.e., Imd).Further, the characteristic (i.e., coefficient) of the current increaserate (kim) is constructed such that when the temperature is low thecoefficient is as indicated by (1) (n.b. the encircled numbers in thediagrams are represented in parenthesis in the specification for reasonsof convenience), and as the temperature rises the coefficient changes to(2) and (3). It is the coefficient setting unit 108 which makes thecoefficient change in accordance with the temperature.

[0073]FIG. 8 is a diagram depicting a result (i.e., an attenuationcharacteristic) of the calculation performed by the maximum currentlimit value calculation unit. Note that this FIG. 8 depicts an examplein which the coefficient does not change. When the detected temperaturechanges, smooth curved lines as shown in FIG. 8 are not produced.

[0074] According to the construction described above, the efficientsetting unit 108 sets the coefficient based on a temperature Tempdetected by means of the temperature sensor 75, and the maximum currentlimit value calculation unit 109 calculates the maximum current limitvalue Limit based on the coefficient set by the coefficient setting unit108. When the handle is turned and held as it is in that position themaximum current limit value (i.e., Limit) attenuates as shown in FIG. 8and limits the motor current. Accordingly, it becomes possible toprevent overheating of the motor 6 and the control device 7.

[0075] Further, when the coefficient is switched as shown in FIG. 7(1)-(3) by means of the coefficient setting unit 108, the attenuationcharacteristics of the maximum current limit value also change as shownin FIG. 8 (1)-(3) such that the motor current is less at a time of hightemperature (3) than at a time of low temperature (1). Accordingly,overheating prevention being adapted to the temperature becomespossible.

[0076] Embodiment 2

[0077] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 2 of the present invention,making reference to the drawings.

[0078] In this Embodiment 2, the control block in FIG. 3 of theabove-described Embodiment 1 is modified as shown in FIG. 9.

[0079] In FIG. 9, the same reference numerals perform the sameoperations as those of FIG. 3, so explanation thereof is omitted, andexplanation will be made of other parts.

[0080] In FIG. 9, 110 is a key switch (signal); 111 is timer formeasuring a duration of time from when the key switch 110 is turned on;and 112 is a control temperature calculation unit (i.e., a controltemperature calculation section) for calculating the control temperature(i.e., Temp) in accordance with the temperature signal 103 and the timer111.

[0081]FIG. 10 is a flow chart depicting an operation of the controltemperature calculation unit.

[0082] Explanation will now be made of operations of the temperaturesignal 103, the key switch 110, the timer 111, and the controltemperature calculation unit 112, making use of FIG. 10.

[0083] In FIG. 10, first, at step 130 a temperature correction valueTempC and the timer are initialized to zero (0).

[0084] Next, at step 131 the signal 103 from the temperature sensor 75is assigned to TempM.

[0085] Next at step 132 the status of the key switch 110 is determined,and if the key switch is off then step 133 is carried out. At step 133the timer 111 for measuring the on time of the key switch is cleared tozero and the procedure advances to step 138.

[0086] In the case when the key switch 110 is on, at step 132 theprocedure splits off to YES, and at step 134 the timer 111 isincremented. Next, at step 135 the value indicated by the timer 111 iscompared against a predetermined value Time1, and in the case when thevalue of the timer 111 is smaller the procedure splits off to NO andadvances to step 138.

[0087] In the case when the timer 111 is greater than the predeterminedvalue Time1, the procedure splits off to YES at step 135 and at step 136the timer 111 is cleared to zero.

[0088] Then, at step 137 the predetermined value T1 is added to thetemperature correction value TempC and the procedure advances to step138.

[0089] At step 138, the temperature correction value TempC is subtractedfrom the TempM which has saved the signal 103 of the temperature sensor75, and the result is assigned to the control temperature Temp.

[0090] After that, at step 139 the control temperature Temp is comparedagainst the predetermined value T2, and in the case when the controltemperature Temp is equal to or greater than the predetermined value T2the procedure splits off to NO and returns to step 131. On the otherhand, in the case when the control temperature Temp is less than thepredetermined value T2 at step 139, the procedure splits off to YES andthe predetermined value T2 is then assigned to the Temp at step 140.

[0091] According to the processing as depicted in FIG. 10, thetemperature correction value TempC is zero immediately after theactivating of the control device 7; therefore, at step 138 the valueTempM detected by means of the temperature sensor 75 is assigned to thecontrol temperature Temp. After that, when the key switch 110 is turnedon the temperature correction value TempC increases by increments of thepredetermined value T1 for each time a predetermined duration of timeTime1 elapses. At step 138 the control temperature Temp drops as thistemperature correction value TempC increases. Note that at steps 139 and140 the control temperature Temp is clipped so that it drops only as faras the predetermined value T2. This situation is depicted by the timingchart of FIG. 11.

[0092] Other parts of FIG. 9 operate similarly to those of Embodiment 1described above. Therefore, the coefficient setting unit 108 sets thecoefficient based on the control temperature Temp calculated by means ofthe control temperature calculation unit 112, and the maximum currentlimit value calculation unit 109 calculates the maximum current limitvalue Limit based on the coefficient set by the coefficient setting unit108.

[0093] In the case of Embodiment 2 of the present invention, theoperations as described above produce effects as follows. In a case whenthe temperature signal 103 detected by means of the temperature sensor75 indicates a high temperature immediately after starting, the controltemperature Temp gradually drops and the maximum current limit isrelaxed in accordance with the elapsing of time from when the key switchwas turned on. That is, even when a temperature inside the passengercompartment of the vehicle (i.e., an ambient temperature) rises, whenthe driver boards the vehicle the control anticipates that the driverwill normally operate the air conditioning or open a window to lower thetemperature inside the passenger compartment.

[0094] In a case when it is necessary due to constructional or othersuch considerations to attach the temperature sensor at a position whichis apart from the place where the temperature must be measured, or alsoin a case when a heat-generating body or an entity which is relativelyhot exists in the vicinity of the temperature sensor and it becomesdifficult to detect the ambient heat of the place where the temperaturemust be measured, a construction such as that of Embodiment 2 enablesthe ambient temperature of the object portion to be predicted and themaximum current limit value to be calculated in accordance therewith.

[0095] Embodiment 3

[0096] Explanation will now be made of the electric power steeringcontrol system according to Embodiment 3 of the present invention, withreference to the drawings.

[0097] In this Embodiment 3, the control block in FIG. 3 of theabove-described Embodiment 1 is modified as shown in FIG. 12.

[0098] In FIG. 12, the same reference numerals perform the sameoperations as those of FIG. 3, so explanation thereof is omitted, andexplanation will be made of other parts.

[0099] In FIG. 12, 150 is the number of engine rotations obtained fromthe engine rotation signal 4 (i.e., the engine rotation detectionsection); 151 is a timer for measuring a duration of time from when thenumber of engine rotations 150 reaches a predetermined value or greater;152 is a control temperature calculation unit for calculating thecontrol temperature Temp in accordance with the temperature signal 103and the timer 151; 153 is a coefficient setting unit for setting acoefficient of the maximum current limit value calculation unit 109based on the control temperature Temp and the temperature signal 103.

[0100] Next, explanation will be made of operations of the temperaturesignal 103, the engine rotation signal 150, the timer 151 and thecontrol temperature calculation unit 152, making use of FIG. 13.

[0101] In FIG. 13, step 132 in FIG. 10 of Embodiment 2 has been changedto step 161. That is, the only change is that the on/off status of thekey switch in FIG. 10 has been changed to the number of engine rotationsbeing greater than/less than the predetermined value NE1.

[0102] Next, explanation will be made of the coefficient setting unit153. The coefficient setting unit 153 has a flag for operating inaccordance with to the temperature signal 103 and operates as shown inFIG. 14.

[0103] First, at step 171 a flag F_mode is cleared to 0. Next, thedetected temperature is checked, and in the case when the detectedtemperature is greater than a predetermined value T3 the proceduresplits off to YES at step 172 and at step 173 the flag F_mode is setto 1. When the temperature drops and is below the predetermined value T3the procedure splits off to NO at step 172.

[0104] Next, at step 174 the detected temperature and the predeterminedvalue T4 are compared, and if the detected temperature is greater thanthe predetermined value T4 then the procedure splits off to NO andreturns to step 172. In the case when the detected temperature is lessthan the predetermined value T4 the procedure splits off to YES at step174, and at step 175 the flag F_mode is cleared to 0, and thereafter,steps 172 to 175 are repeated.

[0105] At this point, when the predetermined values T3 and T4 are setsuch that T3>T4, when the temperature rises and exceeds T3 the flagF_mode is set to 1, and once it is so set, the flag F_mode is maintainedat 1 until the temperature becomes less than the predetermined value T4,and when the temperature becomes less than the predetermined value T4the flag F_mode is cleared to 0.

[0106] The operations of FIGS. 13 and 14 are depicted as a timing chartas shown in FIG. 15.

[0107] Further, in a case when the flag F_mode is 0, the coefficientsetting unit 153 sets the coefficients (1)-(3) in FIG. 16 in accordancewith the control temperature Temp. When the control temperature is highthe coefficient (3) is selected, and as the control temperature dropsthe coefficient is switched from (3) to (2) to (1). However, in the casewhen the flag F_mode is 1, the coefficient setting unit 153 isconfigured to select coefficient (4) regardless of the controltemperature Temp. This produces a result of the following operations.

[0108] First, when the control device 7 activates the flag F_mode iscleared to 0 and the temperature detected by means of the temperaturesensor 75 is set to as the control temperature Temp. The flag F_mode is0, therefore, the coefficient setting unit 153 selects a coefficientfrom (1)-(3) in FIG. 16 in accordance with the control temperature Temp.Then, the maximum current limit value calculation unit 119 calculatesthe maximum current limit value Limit based on the coefficient set bythe coefficient setting unit 153 and the motor current Imd, thuslimiting the motor current by means of the current limiting unit 105.

[0109] After that the engine starts, and when the number of enginerotations 150 becomes greater than the predetermined value NE1 the timer151 begins incrementing, and the control temperature Temp decreases byincrements of the predetermined value T1 each time the predeterminedtime duration Time 1 elapses until the control temperature Temp drops tothe predetermined value T2.

[0110] As this takes place, the coefficient selected by the coefficientsetting unit 153 changes, and thus the maximum current limit is relaxed.

[0111] When use continues in this state and the value detected by thetemperature sensor 75 rises due to some cause and exceeds thepredetermined value T3, the flag F_mode is set to 1. When the flagF_mode becomes 1 the coefficient setting unit 153 selects thecoefficient (4) in FIG. 16. This coefficient (4) is the coefficientwhich most quickly limits the motor current, so the rising of thetemperature may be suppressed. Further, when the temperature drops andthe temperature detected by the temperature sensor 75 becomes less thanthe predetermined value T4, the flag F_mode is cleared to 0 and themaximum current limit value calculation which is suitable for theoriginal control temperature Temp is reset again.

[0112] Operations in accordance with Embodiment 3 are as describedabove; therefore, in this embodiment it is forecast that the ambienttemperature (i.e., the temperature inside the passenger compartment)will drop due to operation of the air conditioner or such when thedriver boards the vehicle and starts the engine, and the maximum currentcalculation is performed in accordance with that effect. Accordingly,unnecessary limitation of the current is not performed, so the overallcontrol is pleasant in feeling. Additionally, in a case when thetemperature rises and the temperature detected by target currenttemperature sensor 75 rises in contrast to the forecast that thetemperature would drop, it is possible to force a switch of the controlcoefficient and urge the system to perform the calculation of themaximum current limit value so as to immediately suppress the rising ofthe temperature.

[0113] Embodiment 4

[0114] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 4 of the present invention,making reference to the drawings.

[0115]FIG. 17 is a control block diagram of Embodiment 4, in which theengine rotation signal 150 of the control block of Embodiment 3 shown inFIG. 12 is changed to a vehicle speed 101.

[0116] With respect to the controls, only one a part thereof has beenchanged. That is, FIG. 18 is a flow chart of this Embodiment 4 in whichthe step 161 of Embodiment 3 shown in FIG. 13 has been changed asindicated at step 191.

[0117] Therefore, as for the operations thereof, in Embodiment 3 thetimer begins operating from the time of the starting of the engine;however, in Embodiment 4 the timer only begins operating from the timewhen the vehicle speed 101 becomes greater than a predetermined value ofSP1. Therefore, Embodiment 4 operates as shown in the timing chart ofFIG. 19.

[0118] Operation in accordance with Embodiment 4 are as described above,so when a driver boards and begins to run the vehicle, a drop in theambient temperature is predicted and a calculation of the maximumcurrent limit is performed in accordance with this result. Accordingly,unnecessary limitation of the current is not performed, so the overallcontrol is pleasant in feeling. Additionally, in a case when thetemperature rises in contrast to the prediction of a drop in temperaturesuch that the temperature detected by the temperature sensor 75 rises,it is possible to force a switch of the control coefficient, and causethe calculation of a maximum current limit value so as to immediatelysuppress the rise in the temperature.

[0119] Embodiment 5

[0120] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 5 of the present invention,making reference to the drawings.

[0121]FIG. 20 is a control block diagram of Embodiment 5, in which theengine rotation signal 150 of the control block of Embodiment 3 shown inFIG. 12 is changed to a torque 102.

[0122] Next, explanation will be made of the controls making referenceto FIG. 21 and FIG. 22. At step 201 in FIG. 21, the control temperaturecalculation unit 152 clears the flag F_TRQ to zero. Next, at step 202 anabsolute value of the torque signal 102 and a predetermined value TRQ1are compared against each other, and in the case when the absolute valueof the torque is less than the predetermined value TRQ1 the proceduresplits off to NO and returns to step 202. In the case when the absolutevalue of the torque is greater than the predetermined value TRQ1 theprocedure splits off to YES, and at step 203 the flag F_TRQ is set to 1and the procedure returns to step 202.

[0123] Next, explanation will be made of the flow chart of FIG. 22. InFIG. 22, step 161 of the flow chart of FIG. 13 of Embodiment 3 has beenchanged to step 204.

[0124] Performing of control as depicted in FIG. 21 and FIG. 22 enablesthe following operation. Immediately after the control device 7 isactivated the flag F_TRQ is in the cleared state of zero, so at step 204the procedure splits off to NO and the timer 151 is turned to thecleared state of zero. After that the driver steers the handle 1, andwhen the absolute value of the torque becomes greater than thepredetermined value TRQ1 the flag T_TRQ is set to 1 at step 203.

[0125] Once the flag F_TRQ is set to 1, the flag F_TRQ remains set at 1thereafter even if the absolute value of the torque drops below thepredetermined value of TRQ1. When the flag F_TRQ is set to 1 theprocedure splits off to YES at step 204, and the timer 151 performs itsincremental operation. This enables operations as depicted in the timingchart of FIG. 23.

[0126] Operations according to Embodiment 5 are as described above;therefore, when the driver boards the vehicle, once he or she steers thehandle a drop in the ambient temperature is predicted, and thecalculation of the maximum current limit is performed in accordance withthis result. Accordingly, unnecessary limitation of the current is notperformed, so the control is pleasant in feeling. Additionally, in acase when the temperature rises in contrast to the prediction of a dropin temperature such that the temperature detected by the temperaturesensor 75 rises, it is possible to force a switch of the controlcoefficient, and cause the calculation of a maximum current limit valueso as to immediately suppress the rise in the temperature.

[0127] Embodiment 6

[0128] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 6 of the present invention,making reference to the drawings.

[0129]FIG. 24 is a control block diagram of Embodiment 6, in which theengine rotation signal 150 of the control block of Embodiment 3 shown inFIG. 12 is changed to a motor current Imd.

[0130] Next, explanation will be made of operations making reference toFIG. 25 and FIG. 26. At step 211 in FIG. 25, the control temperaturecalculation unit 152 clears the flag F_Im to zero. Next, at step 212 themotor current Imd obtained through the timer 151 and a predeterminedvalue Imd1 are compared against each other, and in the case when motorcurrent Imd is less than the predetermined value Imd1 the proceduresplits off to NO and returns to step 212. In the case when the motorcurrent Imd is greater than the predetermined value Imd1 the proceduresplits off to YES, and at step 213 the flag F_Im is set to 1 and theprocedure returns to step 212.

[0131] Next, explanation will be made of the flow chart of FIG. 26. InFIG. 26, step 161 of the flow chart of FIG. 13 of Embodiment 3 has beenchanged to step 214.

[0132] Performing of control as depicted in FIG. 25 and FIG. 26 enablesthe following operation. Immediately after the control device 7 isactivated the flag F_Im is in the cleared state of zero, so at step 214the procedure splits off to NO and the timer 151 is turned to thecleared state of zero. After that the driver steers the handle 1, andwhen the motor current becomes greater than the predetermined value Imd1the flag F_Im is set to 1 at step 213. Once the flag F_Im is set to 1,the flag F_Im remains set at 1 thereafter even if the motor currentdrops below the predetermined value Imd1.

[0133] When the flag F_Im is set to 1 the procedure splits off to YES atstep 214, and the timer performs its incremental operation. Accordingly,this enables operations as depicted in the timing chart of FIG. 27.

[0134] Operations according to Embodiment 6 are as described above;therefore, when the driver boards the vehicle, once he or she steers thehandle and the current is passed to the motor 6, a drop in the ambienttemperature is predicted, and the calculation of the maximum currentlimit is performed in accordance with this result. Accordingly,unnecessary limitation of the current is not performed, so the controlis pleasant in feeling. Additionally, in a case when the temperaturerises in contrast to the prediction of a drop in temperature such thatthe temperature detected by the temperature sensor 75 rises, it ispossible to force a switch of the control coefficient, and cause thecalculation of a maximum current limit value so as to immediatelysuppress the rise in the temperature. Further, in Embodiment 6 thedetected current of Imd is used for the motor current; however, anequivalent effect may be obtained by using the target current Im1 or Imtshown in FIG. 3 as well.

[0135] Embodiment 7

[0136] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 7 of the present invention,making reference to the drawings.

[0137] The control block diagram of this Embodiment 7 is the same as thediagram used in connection with the above-mentioned Embodiment 1.

[0138] In operation, as well, Embodiment 7 differs from Embodiment 1only with respect to temperature detection 103.

[0139] Explanation will be made of the temperature detection, makingreference to FIG. 28. In FIG. 28, when the control device 7 is activatedthe coefficient setting unit 108 first saves as the Temp the temperaturedetected at step 221. After that, this detected temperature Temp whichhas been saved is then held. This manner of construction enables thecoefficient setting unit 108 to set the coefficient based on thetemperature at the time of the activation of the control device 7.

[0140] A construction such as the one described above enables thefollowing effects. The control device 7 has each of the circuits shownin FIG. 2 other than the motor drive circuit 77 built therein. Due tothis, when an electrical power source is turned on for the controldevice 7 the temperature of the control device 7 rises even if the motorcurrent is not flown (hereinafter, this is referred to asself-generation of heat). When the temperature sensor 75 is installed tothe inside portion of the control device 7 the temperature detected bythe temperature sensor 75 rises due to the self-generation of heat, andit becomes impossible to accurately detect the ambient temperature.

[0141] According to Embodiment 7 the coefficient is set using thetemperature immediately after activation. Immediately after activationthere is almost no self-generation of heat; therefore, the detectedtemperature is the same as the ambient temperature. Therefore, itbecomes possible to set the coefficient in accordance with the ambienttemperature.

[0142] According to Embodiment 7 the temperature that was measured onceat the time of activation is held; however, it is also possible to holda value of an average temperature during a fixed period of time afteractivation when the influence of the self-generation of heat is small,and the coefficient for the calculation of the maximum current limitvalue may be set according to this held temperature.

[0143] Embodiment 8

[0144] Explanation will be now be made of an electric power steeringcontrol system according to Embodiment 8 of the present invention,making reference to the drawings.

[0145]FIG. 29 is a diagram depicting a construction of a controlapparatus according to Embodiment 8 of the present invention. In thisdiagram the same reference numerals as those in FIG. 2 refer to partswhich are the same as in Embodiment 1; therefore, explanation will bemade of parts other than these.

[0146] In FIG. 29 reference numeral 80 is a power circuit (a part of apower supply holding unit).

[0147]FIG. 30 is a diagram depicting an interior construction of thecurrent according to Embodiment 8.

[0148] In FIG. 30, Q1, Q2 and Q3 are transistors, and 81 is a 5-voltpower circuit for generating a steady 5 volts of voltage from a batteryvoltage VB (12 volts) obtained through the transistor Q3.

[0149] A construction such as that of FIG. 30 enables a key switch 5 toturn on, and when the transistor Q1 is turned on or the transistor Q2 isturned on by means of a signal (VCONT) outputted from the micon 79, thetransistor Q3 is turned on and the battery voltage VB is supplied to the5-volt power circuit 81. Accordingly, it becomes possible for the 5-voltpower circuit 81 to supply 5 volts of electrical voltage Vcc to themicon 79.

[0150] Next, explanation will be made of controls of the power circuit80, making reference to the flow chart of FIG. 31. In this figure, atstep 231 the power supply holding unit (not shown; i.e., the part of thepower supply holding section) inside the micon 79 performs an on/offdetermination of the key switch 5 based on information outputted fromthe key switch I/F circuit in FIG. 29. If the key switch 5 is on, thenthe procedure splits off to YES at step 231. After splitting off to YESat step 231, the VCONT signal being outputted from the micon 79 is setto high at step 234 and the transistor Q2 is turned on. After that theprocedure returns to step 231.

[0151] On the other hand, when the key switch 5 is off at step 231 theprocedure splits off to NO. Next, at step 232 the value detected by thetemperature sensor 75 and a predetermined value T5 are compared againsteach other, and in the case when the detected temperature is greaterthan the predetermined value T5 the procedure splits off to YES and thetransistor Q2 is turned on at step 234. In the case when the detectedtemperature is less than the predetermined value T5 the procedure splitsoff to NO at step 232 and the transistor Q2 is turned off at step 233.After that, the procedure returns to step 231 and the same processing isrepeated.

[0152] According to the construction described above, first, when thekey switch 5 is turned on the transistor Q1 is turned on, and due tothis the transistor Q3 is turned on; therefore, the battery voltage VBis supplied to the 5-volt power circuit 81, and when the 5-voltelectrical power source Vcc is supplied to the micon 79 the micon 79 isactivated. When the micon 79 is activated the processing depicted inFIG. 31 is carried out. At this point, the key switch 5 is in the onstate, so according to the processing in FIG. 31 the transistor Q2 isturned on. Next, when the key switch 5 is turned off the transistor Q1is turned off; however, the micon 79 has already turned the transistorQ2 on, so the transistor Q3 is in the on state. Since the transistor Q3is in the on state the power supply is provided to the micon 79 and itis possible for the micon 79 to continue operating.

[0153] In other words, after the key switch 5 is turned off, the powersupply holding section, which is constructed of the power circuit 80 andthe power supply holding unit inside the micon 79, holds the electricalsource Vcc (5 volts) for the micon 79 until the temperature detected bythe temperature sensor 75 drops below the predetermined value T5.

[0154] Although detailed depiction thereof is not made, it is normal tostop the motor drive after the key switch is turned off, and therefore,the temperature of the control device 7 drops. When the temperature ofthe control device 7 drops the temperature detected by the temperaturesensor 75 also drops accordingly. When this temperature drops below thepredetermined value T5 the procedure splits off to NO at step 232 ofFIG. 31, and at step 233, Q2 is turned off. The key switch 5 is alreadyturned off and the transistor Q1 is already in the off state; therefore,when the transistor Q2 turns off this causes the transistor Q3 to turnoff. The power supply to the micon 79 is stopped and the control device7 stops completely.

[0155] When Embodiment 8 is joined together with Embodiment 3, forexample, the following effects are obtained. According to Embodiment 3,when the engine starts it is considered that the driver has boarded thevehicle, and the process is performed for gradually lowering the controltemperature Temp. Accordingly, even in the case when the temperaturedetected by the temperature sensor 75 is high the control temperatureTemp is dropping. Therefore, the limit set by the maximum current limitvalue calculation is relaxed, and good electrical power steering may berealized. However, when the key switch 5 is turned off while thetemperature detected by the temperature sensor 75 is still high and theelectrical source of the control device 7 is cut off and then the systemis immediately reactivated, the micon 79 performs processing once againfrom the beginning. Accordingly, control is started once again beginningwith the high temperature detected by the temperature sensor 75, whichis not desirable.

[0156] However, when joined together with Embodiment 8, the controldevice 7 continues performing control when the temperature detected bythe temperature sensor 75 is still high after the key switch is turnedoff. Even if the key switch is turned off and then immediately turnedon, the control device 7 does not perform processing again from thebeginning. Therefore, the maximum current limit value calculation is notperformed using a high temperature. Additionally, after the key switchis turned off, if the temperature detected by the temperature sensor 75drops and goes below the predetermined value, T5 then the electricalsource of the control device 7 is cut off. When it is reactivated thenext time the processing is performed again from the beginning; however,at this point the temperature of the temperature sensor 75 has dropped,so a current limiting value calculation is not performed using a hightemperature. In this way in Embodiment 8 it is possible to avoidperforming a calculation of the motor current limit value at a hightemperature, so good electrical power steering may be realized.

[0157] Embodiment 9

[0158] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 9 of the present invention,making reference to the drawings.

[0159] In Embodiment 9 the flow chart in FIG. 31 pertaining toEmbodiment 8 is changed to FIG. 32.

[0160] Explanation will now be made of FIG. 32. At step 241 theelectrical source holding unit inside the micon 79 determines whetherthe key switch 5 is ON or OFF based key switch information inputted fromthe key switch I/F circuit 74 in FIG. 29, and in the case when the keyswitch is on the procedure splits off to NO. Next, at step 242 the keyoff timer is cleared to zero and at step 243 the VCONT signal (high) isoutputted so as to turn on the transistor Q2. Then the procedure returnsto step 241.

[0161] On the other hand, in the case when the key switch 5 is turnedoff, the procedure splits off to YES at step 241 and at step 244 the keyoff timer performs increments. Next, at step 245 the key off timer ischecked, and when the key off timer is below a fixed time Time2 theprocedure splits off to NO. At step 246 the temperature detected by thetemperature sensor 75 and the predetermined value T5 are comparedagainst each other, and when the detected temperature is greater thanthe predetermined value T5 the procedure splits off to NO and advancesto step 243.

[0162] When the key off timer is greater than the predetermined valueTime 2 at step 245 or when the temperature is less than thepredetermined value T5 at step 246 the procedure advances to step 247,and the VCONT signal (low) is outputted so as to turn the transistor Q2off. After that the process returns to step 241.

[0163] According to the process described above, when the temperature isgreater than the predetermined value T5 after the key switch is turnedoff the transistor Q2 is turned on and the power supply is provided tothe micon 79, and accordingly, it becomes possible to continue control.Further, when the temperature drops and goes below the predeterminedvalue T5 the transistor Q2 is turned off, the provision of the powersupply to the micon 79 is stopped and the control device 7 may becompletely stopped; therefore, it is possible to obtain an effectequivalent to that of Embodiment 8.

[0164] Additionally, according to Embodiment 9, when a fixed duration oftime elapses after the key off timer is used to turn the key switch off,according to step 247 the transistor Q2 is turned off and enables thecontrol device 7 to be turned off. Because of this process, when thetemperature sensor 75 fails and a state continues in which the detectedtemperature value exceeds the predetermined value T5 it is stillpossible to cut off the electrical source and stop the control device 7when the predetermined duration of time elapses. Therefore, even if thetemperature sensor 75 fails it is possible to prevent the battery fromgoing dead.

[0165] Embodiment 10

[0166] Explanation will now be made of an electric power steeringcontrol system according to Embodiment 10 of the present invention,making reference to the drawings.

[0167] In Embodiment 10 the flow chart of FIG. 13 for Embodiment 3 isaltered as depicted in FIG. 33. The reference numerals in FIG. 33 whichare the same as those in FIG. 13 indicate processes which are the sameas Embodiment 3. Note that Embodiment 10 may be applied not only toEmbodiment 3, but also to Embodiments 1 and 2.

[0168] Explanation will be made of the points in FIG. 33 which aredifferent from FIG. 13. First, after the control device 7 is activated,at step 130 the control temperature calculation unit 152 performsinitialization of a corrected temperature value TempC and of the timer,and then at step 251 clears an activation timer to zero. Next, at step252 a value equal to the temperature detected by the temperature sensor75 less a correction amount COR is substituted for TempM. Next, at step253 the above-mentioned activation timer is incremented. Thereafter, theprocessing from step 161 to step 140 is the same as in Embodiment 3described above, and after that, the process returns to step 252 andrepeats the same processing.

[0169] Next, explanation will be made of the correction amount COR ofstep 252. This correction amount COR is a value which changes with time,as shown in FIG. 34, and this time is the time measured by theabove-mentioned activation timer.

[0170] Typically, when the control device 7 is activated the temperaturerises with time due to the energy consumed by the circuit inside thecontrol device 7, even if the motor current is not transmitted. Due tothis self-generation of heat, there develops a discrepancy between thetemperature at the portion which needs to be measured and the detectedtemperature; therefore, calculation of the appropriate maximum currentlimit value becomes difficult. However, according to Embodiment 10, bysetting the characteristics of the correction amount COR in FIG. 34 inaccordance with the characteristics of the self-generation of heat itbecomes possible to make a calculation of an appropriate maximum currentlimit value.

What is claimed is:
 1. An electric power steering control systemcomprising: a motor for adding steering assistance force to a steeringsystem; a steering force detection section for detecting steering forceof the steering system; a motor current determination section fordetermining a motor current based on at least the steering forcedetected by means of the steering force detection section; a temperaturedetection section for detecting an ambient temperature; a coefficientsetting section for setting a coefficient in accordance with a detectedtemperature obtained by means of the temperature detection section; amotor current detection section for detecting a current being passed tothe motor; a maximum current limit value calculation section forcalculating a maximum current limit value based on a current detected bythe motor current detection section and the coefficient set by thecoefficient setting section; a current limiting section for selectingthe smaller value between the motor current determined by the motorcurrent determination section and the maximum current limit valuecalculated by the maximum current limit value calculation section, andoutputting this as a target current; and a motor current control sectionfor passing the target current to the motor in such a way that the motorcurrent is equal to the current detected by the motor current detectionsection.
 2. An electric power steering control system comprising: amotor for adding steering assistance force to a steering system; asteering force detection section for detecting steering force of thesteering system; a motor current determination section for determining amotor current based on at least the steering force detected by means ofthe steering force detection section; a temperature detection sectionfor detecting an ambient temperature; a timer for measuring time fromwhen predetermined conditions are established; a control temperaturecalculation section for calculating a control temperature based on thetemperature detected by the temperature detection section and the timemeasured by the timer; a coefficient setting section for setting acoefficient based on the control temperature calculated by the controltemperature calculation section; a motor current detection section fordetecting a current being passed to the motor; a maximum current limitvalue calculation section for calculating a maximum current limit valuebased on a current detected by the motor current detection section andthe coefficient set by the coefficient setting section; a currentlimiting section for selecting the smaller value between the motorcurrent determined by the motor current determination section and themaximum current limit value calculated by the maximum current limitvalue calculation section, and outputting this as a target current; anda motor current control section for passing the target current to themotor in such a way that the motor current is equal to the currentdetected by the motor current detection section.
 3. An electric powersteering control system comprising: a motor for adding steeringassistance force to a steering system; a steering force detectionsection for detecting steering force of the steering system; a motorcurrent determination section for determining a motor current based onat least the steering force detected by means of the steering forcedetection section; a temperature detection section for detecting anambient temperature; a timer for measuring time from when predeterminedconditions are established; a control temperature calculation sectionfor calculating a control temperature based on the temperature detectedby the temperature detection section and the time measured by the timer;a coefficient setting section for setting a coefficient based on thecontrol temperature calculated by the control temperature calculationsection and the temperature detected by the temperature detectionsection; a motor current detection section for detecting a current beingpassed to the motor; a maximum current limit value calculation sectionfor calculating a maximum current limit value based on a currentdetected by the motor current detection section and the coefficient setby the coefficient setting section; a current limiting section forselecting the smaller between the motor current determined by the motorcurrent determination section and the maximum current limit valuecalculated by the maximum current limit value calculation section, andoutputting this as a target current; and a motor current control sectionfor passing the target current to the motor in such a way that the motorcurrent is equal to the current detected by the motor current detectionsection.
 4. An electric power steering control system according to claim2, wherein the predetermined condition is that the key switch is on. 5.An electric power steering control system according to claim 2, furthercomprising an engine rotation detection section for detecting the numberof engine rotations, wherein the predetermined condition is that thenumber of engine rotations detected by the engine rotation detectionsection is greater than a predetermined value.
 6. An electric powersteering control system according to claim 2, further comprising avehicle speed detection section for detecting a vehicle speed, whereinthe predetermined condition is that the vehicle speed detected by thevehicle speed detection section is above a predetermined value.
 7. Anelectric power steering control system according to claim 2, wherein thepredetermined condition is that the steering force detected by thesteering force detection section is greater than a predetermined value.8. An electric power steering control system according to claim 2,wherein the predetermined condition is that the motor current is greaterthan a predetermined value.
 9. An electric power steering control systemaccording to claim 1, wherein the coefficient setting unit sets thecoefficient in accordance with a detected temperature at the time ofactivation obtained by means of the temperature detection section. 10.An electric power steering control system according to claim 1, furthercomprising a power supply holding section for holding a power supplyuntil the temperature detected by the temperature detection sectiondrops below a predetermined value after the key switch is turned off.11. An electric power steering control system according to claim 1,further comprising a power supply holding section for holding a powersupply until the temperature detected by the temperature detectionsection drops below a predetermined value after the key switch is turnedoff, or until a duration of time having elapsed since the key switch wasturned off is measured and the elapsed duration of time becomes greaterthan a predetermined duration of time.
 12. An electric power steeringcontrol system according to claim 1, wherein the control temperaturecalculation section calculates the control temperature based on atemperature that is the temperature detected by the temperaturedetection section and corrected by a correction amount set in accordancewith characteristics of self-generation of heat, and the duration oftime measured by the timer.